Mn--Zn ferrites are much used as materials for transformers for various communication appliances, household and industrial appliances, etc. The temperature at which transformers in conventional switching power supplies are used falls between 60 and 100.degree. C., and the switching frequency for the power supplies falls between 10 and 100 kHz. The minimum core loss in transformers is within the practical temperature range, and various studies of additive addition, element substitution, firing condition change and others have heretofore been made so as to lower the core loss in transformers within the practical temperature range (see Japanese Patent Application Laid-Open (JP-A) Hei-8-169756, etc.). With the recent tendency toward small-sized and lightweight switching power supplies, high frequency has become used for switching frequency. Low core loss materials for a frequency falling between 500 kHz and 1 MHz or so have been developed (see JP-A Hei-8-148323, etc.).
However, since transformers generate heat by themselves and are used in high-temperature conditions, the temperature of the transformer core being actually driven is often high to fall between 80 and 110.degree. C.
Ferrites for power supplies of that type have been proposed in JP-A Hei-3-141612 and Hei-7-297020. The Mn--Zn ferrites proposed therein have an essential component of ZnO of being not smaller than 10 mol %, and contains an additive of Nb.sub.2 O.sub.5 in the former (Hei-3-141612) or Nb.sub.2 O.sub.5 combined with ZrO.sub.2 in the latter (Hei-7-297020) and additionally SnO.sub.2 and TiO.sub.2 in an amount of not smaller than 300 ppm, to thereby lower the core loss in those ferrites.
However, it is still desired that magnetic oxide materials shall satisfy not only low core loss but also high saturation magnetic flux density therein so as to realize small-sized and efficient transformers within the practical temperature range noted above.
Mn--Zn ferrites having a ZnO content of not smaller than 10 mol % could realize reduced core loss therein, but could not realize increased saturation magnetic flux density within the practical temperature range noted above, especially at a temperature falling between 100 and 110.degree. C.
In this connection, the core loss reduction in Mn--Zn ferrites could be realized by element substitution of adding SnO.sub.2 and TiO.sub.2 to the ferrites. However, no magnetic oxide materials are known capable of realizing increased saturation magnetic flux density within the practical temperature range noted above.
On the other hand, where the practical temperature range for magnetic oxide materials is broad, for example, it falls between 20 and 120.degree. C., the materials are indispensably required to satisfy low core loss therein. However, if the saturation magnetic flux density in the materials varies, depending on the temperature at which the power supplies comprising the materials are used, and, as a result, if the permeability of the materials is thereby varied, the inductance of the materials shall vary, depending on the ambient temperature even though the magnetic field around the materials does not vary. For these reasons, therefore, when the materials are used for power smoothing, the ripple in the outputted power will vary, thereby causing the problem that the power-outputting characteristics of the power supplies comprising the materials vary.
Mn--Zn ferrites having a ZnO content of not smaller than 10 mol % could realize reduced core loss therein, but the temperature-dependent variation in the saturation magnetic flux density in them is large within the practical temperature range noted above.
For the reasons mentioned above, it is desired to develop ferrites and transformers suitable to use at high temperatures and also in a broad temperature range including high temperatures.
On the other hand, desired are transformers in which the core loss is low at their driving temperatures under the conditions under which they may generate a small quantity of heat by themselves. For such transformers in that situation, it is still desired to lower the high-frequency core loss in them so as to meet the requirement of small-sized and lightweight switching power supplies. The driving temperature for transformers generally falls between 50 and 70.degree. C. However, with advance knowledge of the generation of heat in transformers, ferrites for transformers may be so designed that the core loss in the transformers comprising them could be the lowest within a temperature range falling between 80 and 100 C. (see JP-A Hei-3-141612, Hei-7-297020, Hei-8-169756, etc.). Of those patent publications, JP-A Hei-8-169756 discloses a low core loss Mn--Zn ferrite core which consists essentially of from 25 to 40 mol % of MnO and from 6 to 25 mol % of ZnO with the balance of Fe.sub.2 O.sub.3 and contains, as side components, from 0.002 to 0.040 wt. % of SiO.sub.2 and from 0.02 to 0.20 wt. % of CaO and in which the side component elements are segregated to be in the grain boundaries in such a manner that the half width of their concentration distribution is not larger than 10 nm. They say in JP-A Hei-8-169756 that the side components may further include Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, ZrO.sub.2 and V.sub.2 O.sub.5. The composition of the samples actually demonstrated in their Examples was so designed that the core loss in them could be the lowest at 90.degree. C. Briefly, the composition of the essential components in those samples is composed of 53.5 mol % of Fe.sub.2 O.sub.3, 34.5 mol % of MnO and 12.0 mol % of ZnO.
As in JP-A Hei-8-169756, the ferrite is so designed that the core loss therein could be the lowest at temperatures higher then its driving temperature. However, under the conditions under which transformers may generate a small quantity of heat by themselves, it is necessary that the core loss in the transformers is lower within a temperature range of from 50 to 70.degree. C. within which the transformers are driven.
To meet the requirement, JP-A Hei-8-191011 discloses an Mn--Zn--Co ferrite in which the core loss is reduced within the driving temperature noted above. The additive oxides that may be in the ferrite include SiO.sub.2, CaO, ZrO.sub.2 and Ta.sub.2 O.sub.5.
However, the Mn--Zn--Co ferrite in JP-A Hei-8-191011 is still problematic in that the core loss therein within the driving temperature range that falls between 50 and 70.degree. C. could not be reduced to a satisfactory level, and therefore requires further studies on it.